基于流域水环境模型评估重金属面源污染负荷

为定量评估重金属面源污染负荷，以分布式流域水环境模型SWAT模型为基础，借鉴输出系数模型，考虑溶出率影响，发展了一个重金属面源污染模型—SWAT-MT模型. SWAT-MT模型模拟重金属在流域内的大气沉降、地表径流冲刷、土壤和河道中的运移过程，结构较为简单，兼具机理模型和输出系数模型的优点. 本文应用SWAT-MT模型评估了湘江株洲段一个子流域的铅面源负荷，并通过敏感性分析和蒙特卡洛模拟识别了铅面源模拟的敏感参数，量化了模型输出中的不确定性. 结果表明:①研究区铅的面源侵蚀量为2.09 t/a，侵蚀强度为5.27 kg/(km2·a). 铅侵蚀强度时空差异明显，汛期(4—7月)雨水集中，铅的侵蚀量大. 侵蚀强度高的地方集中在流域下游，城镇用地的侵蚀强度约为流域平均水平的2倍. ②流域出口的铅负荷为0.79 t/a，约60%的铅侵蚀量滞留在河道，说明沉降是河道运移的主要过程. ③蒙特卡洛模拟结果显示，模型输出存在显著的不确定性， 95%置信区间下，铅的面源侵蚀量为1.62～2.59 t/a，流域出口负荷为0.50～1.16 t/a；为降低模拟不确定性并提高模型可靠性，应重点关注城市地表积尘的铅浓度、铅溶出率和河道铅沉降速率等敏感参数的取值. 研究显示，通过合理的参数赋值，SWAT-MT模型能够提供较为可靠的重金属面源负荷估算结果，为重金属污染综合防治工作提供科学工具. 

Assessment of Non-Point Source Load of Heavy Metals Using a Watershed Water Quality Model

A SWAT-MT model was developed based on the classic distributed watershed water quality model, SWAT, to evaluate the non-point source (NPS) load of heavy metals. The SWAT-MT model simulates the atmospheric deposition, surface runoff erosion, soil migration and river transport of heavy metals in the watershed. The model refers to the export coefficient model and uses a dissolution rate to describe the leaching of heavy metals in surface runoff. It retains the advantages of the process-based mechanism model and the export coefficient model. The SWAT-MT model was used to evaluate the NPS load of Pb in the subbasin of the Xiangjiang River in Zhuzhou City. Sensitivity analysis and uncertainty analysis were carried out. The results show that: (1) The erosion load of Pb in the study area is 2.09 t/a and the erosion intensity is 5.27 kg/(km2·a), which is consistent with the results in the literature. The erosion intensity of Pb exhibits significant spatio-temporal variability. During the flood season (April-July), there is a lot of rainfall, resulting in a lot of Pb erosion. The areas with high erosion intensity are mainly located in the downstream areas, and the erosion intensity of Pb in urban areas is approximately 2 times of the average level. (2) The Pb load at the watershed outlets is 0.79 t/a. Approximately 60% of the eroded Pb is retained in the rivers, indicating that sedimentation dominates the river transport of Pb. (3) Monte Carlo simulation indicates that the modeling results have significant uncertainty. Under 95% confidence interval, the amount of eroded Pb is 1.62-2.59 t/a, and the Pb load at the watershed outlets is 0.50-1.16 t/a. To reduce the uncertainty and improve the reliability of the model, modelers should pay attention to three sensitive parameters, namely, the Pb concentration in urban surface sediments, the dissolution rate of Pb, and the sedimentition rate of Pb in streams. This study shows that SWAT-MT model can provide reliable NPS load estimation of heavy metals and serve as a scientific tool for comprehensive prevention and control of heavy metal pollution after the parameter values are determined properly.